Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the reaction product is water. Such fuel cells generally employ a membrane electrode assembly ("MEA") consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrodes formed of porous, electrically conductive sheet material, typically carbon fiber paper. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes to an external load.
At the anode, the fuel permeates the porous electrode material and reacts at the catalyst layer to form cations, which migrate through the membrane to the cathode. At the cathode, the oxygen-containing gas supply reacts at the catalyst layer to form anions. The anions formed at the cathode react with the cations to form a reaction product.
In electrochemical fuel cells employing hydrogen as the fuel and oxygen-containing air (or substantially pure oxygen) as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of hydrogen ions from the anode to the cathode. In addition to conducting hydrogen ions, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode, oxygen reacts at the catalyst layer to form anions. The anions formed at the cathode react with the hydrogen ions that have crossed the membrane to form liquid water as the reaction product.
Perfluorosulfonic ion exchange membranes, such as those sold by DuPont under its Nafion trade designation, must be hydrated or saturated with water molecules for ion transport to occur. It is generally accepted that, as cations are transported through such perfluorosulfonic membranes, water molecules associated with those cations are also transported. This phenomenon is sometimes referred to as "water pumping" and results in a net flow of water from the anode side of the membrane to the cathode side. Thus, membranes exhibiting the water pumping phenomenon can dry out on the anode side if water transported along with hydrogen ions (protons) is not replenished. Such replenishment is typically provided by humidifying the hydrogen-containing fuel stream prior to introducing the fuel stream into the cell. Similarly, the oxygen-containing oxidant stream is typically humidified prior to introducing the oxidant stream into the fuel cell to prevent the membrane from drying out on the cathode side. Therefore, fuel cells employing these cation exchange membranes require accumulated water to be removed from the cathode (oxidant) side, both as a result of the water transported across the membrane from the water pumping phenomenon and product water formed at the cathode from the reaction of hydrogen ions with oxygen.
The accumulation of water at the cathode is problematic for several reasons. First, the presence of liquid water in the vicinity of the catalyst layer reduces the accessibility of the catalyst to the reactants, resulting in a reduction in the power of the fuel cell. This phenomenon is sometimes referred to as "flooding" of the catalyst sites. Second, the accumulation of liquid water at the cathode interferes with the permeation of reactants through the cathode to the catalyst, again resulting in a loss of power to the fuel cell. Third, the excessive accumulation of liquid water at the cathode can impart physical changes to the adjacent membrane, causing localized swelling and expansion of the membrane. Conversely, dehydration can cause drying and shrinkage of the membrane, resulting in corresponding mechanical stresses at the electrocatalytic interface.
Conventional water removal techniques generally involve conducting water accumulated at the cathode away from the cathode catalyst layer and toward the oxidant stream exiting the cathode. One conventional water removal technique is wicking, or directing the accumulated water away from the cathode using capillaries incorporated in the cathode. Another conventional water removal technique employs screens or meshes within the cathode to conduct water away from the catalyst layer. Still another conventional water removal technique is to incorporate hydrophobic substances, such as polytetrafluoroethylene ("PTFE"; trade name Teflon), into the cathode sheet material to urge accumulated water away from the cathode. These conventional water removal methods are disadvantageous because:
(1) they require water to be expelled from the membrane/electrocatalytic layer into the cathode's porous structure; PA0 (2) the presence of liquid water restricts the flow of oxidant through the interstices of the porous gas diffusion electrode; and PA0 (3) the presence of liquid water in the oxidant flow channels may restrict the flow of oxidant gas in the channels. PA0 (A) a hydrogen-containing inlet fuel stream; PA0 (B) an oxygen-containing inlet oxidant stream; PA0 (C) a fuel cell stack comprising at least one fuel cell comprising: PA0 (D) a water separator for removing water from the outlet fuel stream to produce a dehumidified fuel stream and a removed water stream. PA0 (A) a hydrogen-containing inlet fuel stream; PA0 (B) an oxygen-containing inlet oxidant stream; PA0 (C) an inlet coolant water stream; PA0 (D) a humidification assembly comprising: PA0 (E) a fuel cell stack comprising: PA0 (F) a heat exchanger for removing heat from the outlet coolant water stream to produce a chilled coolant water stream; PA0 (G) a water separator for removing water from the outlet fuel stream to produce a dehumidified fuel stream and a removed water stream; and PA0 (H) a reservoir for receiving the removed water stream from the water separator and the chilled coolant water stream from the heat exchanger. PA0 (A) a hydrogen-containing inlet fuel stream; PA0 (B) an oxygen-containing inlet oxidant stream; PA0 (C) a humidification assembly comprising: PA0 (D) a fuel cell stack comprising: PA0 (E) a water separator for removing water from the outlet fuel stream to produce a dehumidified fuel stream and a removed water stream; and PA0 (F) a reservoir for receiving the removed water stream from the water separator. PA0 (A) a hydrogen-containing inlet fuel stream; PA0 (B) an oxygen-containing inlet oxidant stream; PA0 (C) an inlet coolant water stream; PA0 (D) a fuel cell stack comprising: PA0 (E) a heat exchanger for removing heat from the outlet coolant water stream to produce a chilled coolant water stream; PA0 (F) a water separator for removing water from the outlet fuel stream to produce a dehumidified fuel stream and a removed water stream; and PA0 (G) a reservoir for receiving the removed water stream from the water separator and the chilled coolant water stream from the heat exchanger.
In systems incorporating water removal at the anode, the water may be drawn through the membrane away from the cathode side and into the anode outlet stream while the water is being formed at the membrane/electrocatalytic interface, leaving the oxidant gas free to diffuse to the active catalyst sites.
In U.S. patent application Ser. No. 07/641,601 filed Jan. 15, 1991, now U.S. Pat. No. 5,260,143 issued Nov. 9, 1993, it was disclosed that a new type of experimental perfluorosulfonic ion exchange membrane, sold by Dow under the trade designation XUS 13204.10, did not appear to significantly exhibit the water pumping phenomenon in connection with the transport of hydrogen ions across the membrane. Thus, the transport of water molecules across the Dow experimental membranes did not appear to be as extensive in the transport of hydrogen ions as in the Nafion-type membranes. This reduction of water pumping in the Dow experimental membranes decreases the accumulation of transported water at the cathode and simplifies the removal of all water, which would normally appear at the cathode, on the anode side. As indicated in the earlier patent application, water removal on the anode side can also be practiced with Nafion-type membranes.
As discussed above, hydrogen ion conductivity through ion exchange membranes generally requires the presence of water molecules. The fuel and oxidant gases are therefore humidified prior to introducing them to the cell to maintain the water saturation of the membranes within the MEAs. Ordinarily, the fuel and oxidant gases are humidified by flowing each gas on one side of a water exchange membrane and by flowing deionized water on the opposite side of the membrane. Deionized water is preferred to prevent membrane contamination by undesired ions. In such membrane based humidification arrangements, water is transferred across the membrane to the fuel and oxidant gases. Nafion is a suitable and convenient humidification membrane material in such applications, but other commercially available water exchange membranes are suitable as well. Other non-membrane based humidification techniques could also be employed, such as exposing the gases directly to water in an evaporation chamber to permit the gas to absorb evaporated water.
It is generally preferred to humidify the fuel and oxidant gases at, or as close as possible to, the operating temperature and pressure of the fuel cell. The ability of gases such as air to absorb water varies significantly with changes in temperature and pressure. Humidification of the air (oxidant) stream at a temperature significantly below fuel cell operating temperature could ultimately dehydrate the membrane. Consequently, it is preferable to integrate the humidification function with the active portion of the fuel cell stack, and to condition the fuel and oxidant streams to nearly the same temperature and pressure as the active section of the stack. In such an integrated arrangement, the coolant water stream from the active section, which is at or near the cell operating temperature, is used as the humidification water stream. Similarly, the fuel and oxidant streams are typically directed via manifolds or headers through the active section to condition each to cell temperature prior to introducing them to the humidification section.
In addition to integrating the coolant water stream of the active section with the humidification water stream of the humidification section, it is also advantageous to integrate the fuel cell product water stream with the coolant stream, and thereby use the product water generated electrochemically in the fuel cell stack to regulate the temperature of the stack. In this regard, the use of product water as the coolant avoids the need to provide a separate external source of coolant fluid, since the water generated by the cell is itself a suitable coolant fluid. The use of product water as the coolant fluid is also advantageous during start-up, when the relatively warm product water stream can be used to bring the active section up to operating temperature.
The use of fuel cell designs and operating conditions that permit the removal of accumulated water in the outlet fuel stream of the anode offers several advantages. In particular, water removal on the anode side permits the operation of a hydrogen/oxygen fuel cell in a "dead-ended" mode on the cathode or oxygen side. That is, the oxygen-containing oxidant stream can be fed to the cathode and consumed substantially completely, producing essentially no outlet stream from the cathode. Dead-ended operation thus eliminates the need for an oxygen recirculation pump. Oxygen recirculation pumps are expensive and difficult to maintain because of the corrosive effects of moist oxygen-containing streams like the humidified oxidant stream circulated through hydrogen/oxygen fuel cells. The elimination of an oxygen recirculation pump from a fuel cell system reduces the overall cost of the system and improves the reliability of the fuel cell because of the reduced possibility for oxygen leakage and fires. Eliminating the oxygen recirculation pump also reduces the parasitic (hotel) load on the fuel cell system, resulting in a higher proportion of the electrical power from the fuel cell system being available for delivery to the external load instead of being consumed by operation of the oxygen recirculation pump.
Removal of accumulated water from the anode side of the fuel cell also offers systems advantages when air is used as the oxidant. As described in more detail below, the removal of water at the anode permits the effective operation of fuel cells at lower air flow rates. The term "stoichiometry" is used to characterize gas flow rates. As used herein, stoichiometry refers to the ratio of the quantity of reactant supplied to the fuel cell to the quantity of reactant consumed by the fuel cell. An H.sub.2 stoichiometry of 1.0 means that the quantity of hydrogen consumed by the fuel cell equals the quantity of hydrogen supplied to the fuel cell. In a fuel cell operated with dead-ended oxygen, the stoichiometry of oxygen would be 1.0, the amount of oxygen supplied to the fuel cell being substantially completely consumed. Likewise, an H.sub.2 stoichiometry of 2.0 means that the quantity of hydrogen supplied to the fuel cell is twice the quantity of hydrogen consumed by the fuel cell.
Low oxidant stream stoichiometry, which is made possible by the removal of water at the anode, reduces the parasitic load required to pressurize the oxidant stream. The power required to pressurize the oxidant stream represents a substantial and significant parasitic load in systems operating with dilute oxidant streams, such as oxygen-containing air. The parasitic load is directly proportional to oxidant stoichiometry, so that a decrease in the oxidant stoichiometry reduces the parasitic load, thereby improving the net power deliverable from the system to an external electrical load.
Recently, efforts have been devoted to identifying ways to operate electrochemical fuel cells using impure hydrogen as the fuel. Fuel cell systems operating on substantially pure hydrogen are generally disadvantageous because of the expense of producing and storing pure hydrogen gas. In addition, the use of liquid fuels is preferable to pure, bottled hydrogen in mobile and vehicular applications of electrochemical fuel cells.
Water removal at the anode also offers systems advantages when impure hydrogen is employed as the fuel source. By proper design of the fuel cell and operating conditions, accumulated water can be removed into the exhaust fuel stream, thereby permitting the operation of such a fuel cell system on dead-ended oxygen or low stoichiometry air with the systems advantages described above.
Accordingly, it is an object of the present invention to provide solid polymer fuel cell systems in which water accumulated at the cathode is removed in the outlet fuel stream of the anode, thereby avoiding recirculation of the oxidant stream.
It is also an object of the invention to provide solid polymer fuel cell systems incorporating water removal on the anode side that can be operated using substantially pure fuel streams, such as bottled hydrogen, as well as impure fuel streams, such as those produced from the conversion of hydrocarbons to hydrogen.
It is a further object of the invention to provide solid polymer fuel cell systems incorporating water removal on the anode side that can be operated using substantially pure oxidant streams, such as bottled oxygen, as well as impure oxidant streams, such as oxygen-containing air.